Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.

這是 安裝完 CodiMD 後在執行定期備份時踩到的坑…在使用 crontab + script 定期備份時，我百思不得其解，為啥我跑出來的 backup.sql 會是空的？ 更讓我不解的是，我手動執行 script 的結果卻是正常的！？？？

這是 CodiMD 安裝的後續…，雖然跟 CodiMD 沒啥關係 XD 這次的主要目的是為 CodiMD 做一個定期備份，另外為了儲存空間上的考量，備份檔案只保留 30 天，一旦超過就刪除資料。

Shell Script 的條件宣告是用 [] ，if 條件後面需接 then ，block 結束後接 fi

在寫 Shell Script 時 ，想確定某檔案是否為空，若為空則想執行其他動作。

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

Given an array nums of n integers and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

Given an array nums of n integers, are there elements a, b, c in nums such that a+ b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Write a function to find the longest common prefix string amongst an array of strings.

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.